The present invention relates to a method and a device for determining a speed variable that represents the speed of a vehicle.
German Published Patent Application No. 197 13 251 concerns a method for determining a value that describes the speed of a vehicle. According to this method, the speeds for at least two wheels are determined. The value that describes the speed of the vehicle is determined as a function of the speed of just one selected wheel. The wheel used to determine the value that describes the speed of the vehicle is chosen at least as a function of an operating status of the vehicle, which is described at least via the speeds of at least two wheels and via a value which is determined as a function of at least those speeds and which represents a value that describes the vehicle""s acceleration. This method may have the disadvantage that just one of the vehicle""s wheels is used to support the reference speed. As a result, if the system makes a change in the wheel that is selected, there may be a change in the reference speed, although the vehicle""s actual speed has not in fact changed.
German Patent No. 44 28 347 C2 apparently concerns a circuit arrangement for determining the speed of a vehicle. Apparently, the circuit arrangement has a fuzzy system into which wheel speed relative difference values relative to the speed determined at an earlier sampling instant are input as input variables. With the help of the fuzzy system, weighting factors for the individual wheel speeds are calculated. The weighted mean of the four wheel speeds is calculated and considered the speed of the vehicle; i.e., the vehicle""s speed is directly considered the weighted mean, no support variable being calculated.
An object of an exemplary method and/or an exemplary device of the present invention is to provide a method and/or a device for determining a speed variable that represents a vehicle""s speed which allows the speed variable to be calculated more precisely.
The exemplary method according to the present invention relates to a method for determining a speed variable that represents a vehicle""s speed. To accomplish this, wheel speed values, each of which describe the speed of a given wheel, are determined. Furthermore, weighting variables for the individual wheel speed values are determined. A support variable is determined via averaging, as a function of the wheel speed values that have been weighted using the weighting variables. The speed variable is determined as a function of the support variable.
The speed variable is determined in an improved manner due to the averaging via which a support variable is determined, and due to the fact that the speed variable is determined as a function of the support variable. Due to the averaging, the speeds of a plurality of wheels are used rather than the speed of just one wheel, the speed of a given wheel being weighted as a function of the wheel""s suitability for determining the speed variable. Because the speed variable is determined as a function of the support variablexe2x80x94this method is known as xe2x80x9csupportingxe2x80x9d the speed variablexe2x80x94further weighting can be carried out.
The following further useful advantages are associated with the exemplary method according to the present invention and the exemplary device according to the present invention:
more effective calculation of vehicle reference speed (reference calculation) in vehicle dynamics control systems such as those described in the automotive technology journal Automobiltechnische Zeitschrift 96, 1994, volume 11, pages 674-689, article entitled, xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [Bosch Vehicle Dynamics Control System];
better reference calculation based on full functionality in all driving situations in extreme all-wheel vehicles having up to 50%/50% drive distribution and limited torque coupling in their differentials;
the algorithm for reference calculation is less sensitive to erroneous input signals that can arise, for example, in the case of pulse wheels subject to tolerance on revolution sensing elements;
reference calculation is more transferable. The exemplary method according to the present invention can be used with any drive design (rear-wheel drive, front-wheel drive, all-wheel drive with center differential, all-wheel drive with Torsen differential, all-wheel drive with viscous coupling differential, etc.) without any changes being necessary. Information as to whether a wheel is driven or not is not required. The system is also suitable for use in off-road vehicles, for maintaining anti-lock brake and drive slip control systems when off road.
Moreover, a further advantage is that reference calculation can still be carried out even if the steering angle signal, the transverse acceleration signal, the yaw rate signal, or the engine torque signal are no longer present, and it can therefore be used in the back-up systems of vehicle dynamics control systems. The only input variables that are needed are the wheel speeds and the admission pressure set by the driver. The input variables that are no longer present can be calculated from the wheel speeds via modeling. This advantage arises from the fact that the algorithm for reference calculation is mainly dependent upon the wheel speeds and their derivatives and to a much lesser extent upon signals Fbij and MMot. Thus the exemplary method according to the present invention can also be used in systems in which only the wheel speeds and the wheel braking force in braking situations are available and only a rough estimate of the yaw rate is available, such as is the case in, for example, the back-up systems of vehicle dynamics control systems.
Wheel stability analysis is carried out via a fuzzy logic system, which is used instead of a crisp logic system such as that described in German Published Patent Application No. 197 13 251. Moreover, the estimated vehicle speed is corrected using a supporting wheel speed, and in addition the order of magnitude of this correction is modified on an ongoing basis.
With the help of a fuzzy approach, a supporting wheel speed is calculated from all four wheel speeds. In addition, the probability value for the suitability of the supporting wheel speed is determined and can then be used to calculate, for example, the coefficients for a Kalman-Bucy filter.
The fact that only half of the fuzzification is taken into account (the complement is not taken into account) and that scaling to a probability value is carried out simplifies the calculations considerably, and also means it is not necessary to carry out defuzzification or to subject fuzzy sets in vectorial form to logic operations. This means less processing capacity and storage capacity are required.